1. Field of the Invention
The present invention relates to a failure diagnostic apparatus for a fuel vapor purge system that purges fuel vapor generated in a fuel tank into an intake system and, to a fuel vapor purge apparatus and a combustion engine having the failure diagnostic apparatus.
2. Description of the Background Art
Vehicles with a fuel tank containing volatile liquid fuel generally have a fuel vapor purge system for purging fuel vapor generated in the fuel tank into an intake system. With such a fuel vapor purge system, the fuel vapor generated in the fuel tank is temporarily adsorbed and collected by a canister connected via a vapor passage to the fuel tank and thereafter purged into an air intake passage of an engine connected via a purge passage to the canister.
Most of fuel vapor purge systems of the above-described type have, with the purpose of ensuring the system reliability, a failure diagnostic apparatus for detecting leakage of fuel vapor due to opening of some hole(s) or crack(s) for example of a path including the fuel tank, vapor passage, canister, and purge passage (this path is hereinafter referred to as “evaporation path”). Such a failure diagnostic apparatus uses an electric pump to generate a pressure difference between the inside and the outside of the evaporation path, measures the pressure within the evaporation path, and compares the measured pressure with a predetermined reference pressure to conduct diagnosis for determining whether or not leakage from the evaporation path occurs.
Japanese Patent Laying-Open No. 2003-269265 discloses a failure diagnostic apparatus for a fuel vapor purge system like the aforementioned one. In consideration of influences of the generation of fuel vapor on the internal pressure of the evaporation path, the failure diagnostic apparatus uses a reference pressure that is generated when a pressure is applied to a reference hole having its diameter equal to the diameter of a hole which will cause an abnormality to be detected, the reference pressure being corrected using a pressure detected in advance when the fuel vapor is generated. The corrected reference pressure is used to determine whether or not leakage from the evaporation path occurs.
The failure diagnostic apparatus disclosed in Japanese Patent Laying-Open No. 2003-269265 can thus be used to improve precision with which whether or not leakage failure of the evaporation path occurs is determined.
On the other hand, in the failure diagnostic apparatus disclosed in Japanese Patent Laying-Open No. 2003-269265, influence on the internal pressure within the evaporation path due to condensation of the fuel vapor in the canister is not taken into account. Specifically, when the fuel vapor is adsorbed by the canister, the fuel vapor is condensed and its volume is reduced. Therefore, the internal pressure within the evaporation path is lowered. If the pressure in the evaporation path is set to a negative pressure at the time of failure diagnosis, the failure diagnostic apparatus disclosed in Japanese Patent Laying-Open No. 2003-269265 misdiagnoses abnormal as normal, because the pressure within the evaporation path is measured lower due to condensation of the fuel vapor in spite of presence of a hole to be detected in the evaporation path.